1. Field of the Invention
This invention relates in general to a process for the treatment of metal scrap or waste and, more particularly, to a process for recycling or converting aluminum scrap or waste into finished articles.
2. Description of the Prior Art
It is known in the art to produce finished articles from metal scrap by compression, heating and extrusion. In one of the prior expedients disclosed, for example, in U.S. Pat. No. 3,626,578, scrap metal such as titanium, zirconium, molybdenum, columbium, tantalum, tungsten, steel, or the like, is compacted into a billet having an average density of at least 70% of the theoretical density of the metal. The compacted billet is heated to a plastic deformation temperature sufficient for hot working and thereafter consolidated by hot extrusion so that the billet is deformed into a dense mass having a density between 90 and 100% of the theoretical complete density of the metal.
For scrap metal such as aluminum, it has heretofore been proposed to heat the aluminum scrap and subsequently consolidate the scrap by extrusion in vacuo.
It has also heretofore been proposed to consolidate the aluminum scrap into useful articles by compression, heating and extrusion. In some cases, the aluminum scrap is crushed and cleaned before it is compressed by mechanical pressure methods.
As is apparent from these prior art expedients, it is possible to compress and heat aluminum scrap before it is extruded and, in some cases, it may be possible to complete the extrusion of the aluminum scrap in an evacuated state. In actual practice, however, recycling of aluminum scrap into usable articles through extrusion presents severe and specific difficulties which are inherent in the nature of the aluminum scrap itself.
It is preferred that, before the extrusion step, the aluminum scrap be compressed to make the density of the scrap conform as closely as possible to the theoretical density of the metal to obtain finished articles of high quality sufficient for commercial use. Nevertheless, difficulties occur since the press required must be massive and consumes a large amount of energy as well as requiring a large mounting space.
A drawback is also encountered in the extrusion step performed under vacuum in that the flow rate of the exhausted air sucked by the vacuum pump per unit time is limited in practice. Consequently, if a vacuum must be created for each extrusion step, the working time ratio of the extrusion press is low.
Moreover, as is well known, aluminum is recognized as a metal having a relatively high consolidation rate. This property would seem to be advantageous in the extrusion of aluminum scrap. However, instead it poses a serious problem in obtaining a substantially void-free, dense and scab-free product in the extrusion step. More specifically, when the individual scrap particles constituting the aluminum scrap are compressed, they are deformed. An increase in the compression pressure causes metallic contact of the scrap particles and their consolidation into metallic bodies. Since, in this instance, metallic union of the individual scrap particles is completed within an extremely short period of time because of the high consolidation rate of the scrap metal, exhausted air trapped between the scrap particles cannot escape out of the metallic bodies. This results in the formation of voids or perforations in the final product. These voids or perforations in the metal cause deterioration of mechanical properties such as strength and corrosion resistance. In addition, because of these defects, bloated portions are formed near the surface of the finished product in the course of the extrusion thereby providing poor surface appearance of the extruded article. These bloated portions will hereinafter be referred to as scabs. When the finished articles contain scabs on their surfaces, the commercial value of the recycled products is significantly reduced.
In summary, it has been theoretically recognized that it is advantageous to perform the extrusion of aluminum scrap in a vacuum or after the compression of the aluminum scrap into a mass with a density close to the theoretical complete density of the aluminum metal. These techniques aid in eliminating the formation of voids or perforations during treatment of the aluminum scrap including compression, heating nd extrusion. However, these expedients have disadvantages as discussed hereinabove in that an increase in energy consumption and design of a large-size apparatus are required.
Moreover, it is extremely difficult to produce void-free and scab-free finished articles using apparatus of limited size with a limited amount of energy consumption. Consequently, there is a need for a new process for treating scrap aluminum whereby the finished product will be free of these defects.